From et's link above:

Models are representations of objects that aid in our understanding. If we were to choose a scale model of the Earth, what objects would make a good miniature Earth? We might be tempted to choose an oblate object to show the true shape, but that would be inaccurate! Since the Earth is so slightly oblate, and the Earth's relief is so insignificant in comparison with its size, the best scale models of the Earth would actually be very round and very smooth. Billiard balls, marbles, ping pong balls, and other smooth spheres are the best representation of the Earth's true appearance.